following competition (that’s another
story), when Tom commented that there
were no good, inexpensive (I think he
used the word cheap) speed controllers
to use with these motors. So, as a group,
we pointed at Ken and insisted that he
design one. Since he also bought some
silver bombers (five, I think), it was hard
for him to refuse the challenge.
I’ll let Ken describe the design
■ PHOTO 4. Parts salvaged from a Bomber.
KEN STARTS THE
I (Ken) bought my Silver Bombers
with the intention of either using just
the motors or turning them into some
kind of tracked robot. A PWM driven
controller is a must to build something
There were no markings on the
unit, so the only way to determine
what kind of current it needed was to
measure it. Using a current limited
power supply, I ran it at six volts and
determined that the current draw was
about 2.5 amps with no load. I measured the winding resistance by carefully rotating the commutator until the
lowest possible reading was obtained.
This value would give a good indication of what the stall current would
be. I measured about .715 ohms.
E=I*R give us 24V/.715 ohms or
33. 5 amps. I’ll take that as 35 amps for
the design. With this in mind, we needed a controller that could safely withstand a 35 amp stall current. I asked
some other club members to test their
motors and they got similar results.
In the interest of full disclosure, I
had already started a project like this
some time ago, but was going for something much more sophisticated. I wanted a microcontroller driven unit with full
quadrature or servo input that was
capable of running some fairly sophisticated PID loops. Knowing this group as
I do, I decided we needed something a
little simpler to start with — a study H-bridge with simple control and as much
protection built in as we could afford.
To give you a little background
on me, I am a hardware/software
designer with more than 35 years
experience in many design venues.
Robots have been in my blood since I
built my first one at age 12. Designing
electronics and software is second
nature to me, but some of the intrica-cies of machining and CNC were a
mystery. That’s why I joined the group
— to trade talents and help others out
if I could. Phil and I regularly collaborate on various robotics projects.
A good design always starts with
research. There is no point re-invent-ing the wheel, and lessons learned by
the efforts of others are invaluable in
producing a good, repeatable design.
I searched the web and looked at
many robotic H-bridge projects. Most
had some aspects of what I was looking for, but none quite put it all together in the professional (and sometimes
quirky) manner I’m accustomed to.
For the purposes of this article, I’ll
dispense with all the design equations
needed to select parts and properly
control the MOSFETs. These are readily
available in many places; a quick search
will ferret them out if you are really
interested in this aspect of the design.
I settled on using the Intersil
HIP4081A driver — a common and
hardy solution with more than enough
drive for a 50A controller. I had talked
to the Intersil rep and he assured me it
was a solid part and that they intended
to support it for some time to come.
Coincidentally, I discovered that
this is the same part used in the
OSMC controller project and many of
the humanoid robot designs in Japan.
Couple this to some low Rds on FETs
and you have the makings of a reliable
The club members all
have different controllers
they are experimenting with.
Some are using Bascom,
others using embedded ‘C’
code. To accommodate this
variety, I chose to provide
the following inputs that
can be driven by any
3. A PWM or brake input. When held
low, motor braking action is initiated.
4. A fan enable pin. The fan can be
turned off to conserve power when
idle. The fan can also be PWM driven
to provide fan speed control.
5. An auxiliary relay drive. Can be used
to totally disconnect the motors or control brakes if using wheelchair motors.
6. A scaled voltage output for measuring the input voltage.
7. A heatsink temperature output
scaled 10 mv/deg C.
All of the inputs and outputs are
isolated (most optically), except the
scaled input voltage. Fully isolating
this would have made the project
complete, but added too much cost.
In addition, since it is opto-isolated, the driver can be powered by
controllers running on 3.3V power
supplies without any level translators.
The block diagram in Figure 1
■ PHOTO 5. Bomber motor.
1. A master disable to
disconnect the FETs.
2. A direction pin.
September 2006 29